A bottle cap is an element that covers the outer periphery of a bottle's neck. The principal function of the cap is to impede the entrance or exit of fluids from the bottle.
To fulfill this function, the cap must be substantially immobilized and pressed onto the extreme periphery of the neck of the bottle. For this, one generally resorts to a subjection technique, consisting of an interaction of the form and plastic characteristics of the cap with the form of the bottle neck.
The twist-off cap is one example. The essence of the cap is the combination of two principles. The first of these is that a plastic liner seals the top of the bottle, and the second is that a surrounding cap presses down on the plastic liner. To be able to secure the cap, one must interact with some resource that is found disposed on the exterior of the bottle.
Another example is the crimp cap that is made of metal and which is mounted around the bottle. The cap is pressed around the neck of the bottle to copy the sinuous form of the bottle's peripheral end.
A bottle cap has previously been defined as an element to close the extreme end of a bottle that impedes the entrance or departure of elements from the bottle. Caps are differentiated between one another by their constituent material, by their form and by the technique used for their fixation on the extreme end of the bottle and their extraction from same.